1. Field of the Invention
The present invention relates to a crystal oscillation circuit, and more specifically to a technology of suppressing the unnecessary oscillation of a quartz oscillator.
2. Description of the Prior Art
Generally, a piezoelectric crystal used for a quartz oscillator can excite the piezoelectric oscillation only at a specific cutting angle for a crystallographic axis. SC-cut and IT cut quartz oscillators are well known as quartz oscillators appropriate for such an oscillator.
However, the SC-cut and II-cut quartz oscillators generate B mode and A mode secondary oscillation (unnecessary mode) near the resonance in the C mode (main mode) as primary oscillation, and at the high frequency side.
Since the value of the crystal impedance (hereinafter referred to as CI for short) at the oscillation in the A mode is larger than that in the C mode, no specific problem occurs. On the other hand, the CI in the B mode is equal to or sometimes smaller than the CI in the C mode. Therefore, when an oscillator is actually produced, there can be the problem that the oscillation is performed in the B mode as the secondary oscillation.
When a quartz oscillator of the SC cut or IT cut is used, it is necessary to suppress the oscillation in the B mode to correctly excite the primary oscillation in the C mode. To attain it, a crystal oscillation circuit for suppressing the secondary oscillation in the B mode has been proposed.
FIG. 1A shows a colpitts crystal oscillation circuit provided with a common B mode suppression circuit. It has the configuration of a serial resonance circuit (by the dotted lines) including an L (coil: inductance device) and a C (capacitor: capacitance device) tuned for the C mode to enable the negative resistance (NR) to have a frequency characteristic. FIG. 1B shows an equivalent circuit of the circuit shown in FIG. 1A. The condition of the colpitts oscillation circuit is that a signal passes in both directions to the resonance circuit (by dotted lines) inserted into the feedback loop of the circuit shown in FIG. 1A.
Thus, the colpitts oscillation circuit that suppresses B mode oscillation and stably oscillates the C mode inserts a serial resonance circuit that resonates at the frequency in the C mode to enable the feedback loop to have an optional frequency.
However, a circuit using a coil and a capacitor cannot positively suppress the oscillation in the B mode.
For example, although the selection characteristic depends on the Q value of a coil, the Q value is 50 at most in the case of a small SMD (surface mount device) type winding coil. Therefore, the B mode cannot be sufficiently suppressed. That is, there is the problem that the negative resistance at the frequency of the B mode in an oscillation circuit remains as the negative state.
For example, according to the Japanese Published Patent Application No. 2002-261546, when there is spurious at a frequency lower than a predetermined resonance frequency in a quartz oscillator incorporated into a crystal oscillation circuit, the oscillation occurs by the spurious. Therefore, a crystal oscillation circuit with suppressed spurious oscillation has been proposed.
Additionally, the Japanese Published Patent Application No. 2000-295037 also proposes the technology of making the absolute value of the negative resistance at the frequency in the B mode smaller than the effective resistance in the B mode while keeping the negative resistance at the frequency in the C mode by adding an inductance, variable reactance, and two chip capacities.
The Japanese Published Patent Application No. H7-7363 proposes the technology of providing an SC-cut quartz oscillator for suppressing spurious for a variation of a colpitts crystal oscillation circuit so that the negative resistance in the B mode can be adjusted by the resonance for the frequency in the B mode.
However, for the correct oscillation only in the C mode, it is necessary to make the negative resistance of the frequency in the B mode a positive value. In the method described in the above-mentioned patent document, it is not possible to make the negative resistance of the frequency in the B mode a positive value.
Furthermore, when a quartz oscillator is used as a trap circuit for a suppression circuit in the B mode as indicated by the Japanese Published Patent Application No. H7-7367, the negative resistance in the B mode can be made positive at an arrow frequency band indicated by the curve A shown in FIG. 2. However, as shown in FIG. 2, it is difficult to correctly make an adjustment because a positive value can be obtained only in a narrow bandwidth in the B mode (for example, 10.9 MHz). In addition, since a quartz oscillator is used, it is costly. Furthermore, the temperature characteristic in the B mode indicates a large variation depending on a temperature change. Therefore, it is difficult to stably apply it in a wide temperature range.
When a coil having a large Q value and a capacitor are used, it is hard to make an adjustment because the pass band width in the C mode is very narrow.
When a ceramic resonator is used, it is hard to make an adjustment because there is much spurious of the ceramic resonator itself.